Montroyal I have to disagree w/your post really. Yanno, I don't mean to state the OBVIOUS, but, it's not her weight that concerns me, but the habits. IF you suspect an eating disorder, i would get her mom to get professional help ....weight has NOTHING to do with it, really. You can be 200lb and have an eating disorder (ever hear of compulsive OVEReating?)--- you can be NORMAL weight and have a disorder, (ever hear of BULIMIA?).......it's the behaviors!!!! You can't always tell from just OUTWARD appearances who is affected, you know! There is usually an underlying cause that extends WAY beyond the eating....and it begs investigation.
For some additional info from eating disorder experts;
Not a plug, but you may find some useful info there.
I wish your family the best. I hope she will make out OK! (I have extensive personal experience w/eating disorders, PM me if you wish for more help).